On Thu, Oct 03, 2024 at 02:03:07PM GMT, Eduard Zingerman wrote:
> test_progs uses glibc specific functions backtrace() and
> backtrace_symbols_fd() to print backtrace in case of SIGSEGV.
>
> Recent commit (see fixes) updated test_progs.c to define stub versions
> of the same functions with attriubte "weak" in order to allow linking
> test_progs against musl libc. Unfortunately this broke the backtrace
> handling for glibc builds.
>
> As it turns out, glibc defines backtrace() and backtrace_symbols_fd()
> as weak:
>
> $ llvm-readelf --symbols /lib64/libc.so.6 \
> | grep -P '( backtrace_symbols_fd| backtrace)$'
> 4910: 0000000000126b40 161 FUNC WEAK DEFAULT 16 backtrace
> 6843: 0000000000126f90 852 FUNC WEAK DEFAULT 16 backtrace_symbols_fd
>
> So does test_progs:
>
> $ llvm-readelf --symbols test_progs \
> | grep -P '( backtrace_symbols_fd| backtrace)$'
> 2891: 00000000006ad190 15 FUNC WEAK DEFAULT 13 backtrace
> 11215: 00000000006ad1a0 41 FUNC WEAK DEFAULT 13 backtrace_symbols_fd
>
> In such situation dynamic linker is not obliged to favour glibc
> implementation over the one defined in test_progs.
>
> Compiling with the following simple modification to test_progs.c
> demonstrates the issue:
>
> $ git diff
> ...
> \--- a/tools/testing/selftests/bpf/test_progs.c
> \+++ b/tools/testing/selftests/bpf/test_progs.c
> \@@ -1817,6 +1817,7 @@ int main(int argc, char **argv)
> if (err)
> return err;
>
> + *(int *)0xdeadbeef = 42;
> err = cd_flavor_subdir(argv[0]);
> if (err)
> return err;
>
> $ ./test_progs
> [0]: Caught signal #11!
> Stack trace:
> <backtrace not supported>
> Segmentation fault (core dumped)
>
> Resolve this by hiding stub definitions behind __GLIBC__ macro check
> instead of using "weak" attribute.
>
> Fixes: c9a83e76b5a9 ("selftests/bpf: Fix compile if backtrace support missing in libc")
>
> CC: Tony Ambardar <tony.ambardar@xxxxxxxxx>
> Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> ---
> tools/testing/selftests/bpf/test_progs.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/test_progs.c b/tools/testing/selftests/bpf/test_progs.c
> index 7846f7f98908..005ff506b527 100644
> --- a/tools/testing/selftests/bpf/test_progs.c
> +++ b/tools/testing/selftests/bpf/test_progs.c
> @@ -20,20 +20,23 @@
>
> #include "network_helpers.h"
>
> +/* backtrace() and backtrace_symbols_fd() are glibc specific,
> + * use header file when glibc is available and provide stub
> + * implementations when another libc implementation is used.
> + */
> #ifdef __GLIBC__
> #include <execinfo.h> /* backtrace */
> -#endif
> -
> -/* Default backtrace funcs if missing at link */
> -__weak int backtrace(void **buffer, int size)
> +#else
> +int backtrace(void **buffer, int size)
> {
> return 0;
> }
>
> -__weak void backtrace_symbols_fd(void *const *buffer, int size, int fd)
> +void backtrace_symbols_fd(void *const *buffer, int size, int fd)
> {
> dprintf(fd, "<backtrace not supported>\n");
> }
> +#endif /*__GLIBC__ */
>
> int env_verbosity = 0;
>
> --
> 2.46.1
>
>
Acked-by: Daniel Xu <dxu@xxxxxxxxx>
